In recent years, X-ray imaging apparatus for performing circulatory organ fluoroscopic radiography have widely been used. The circulatory organ fluoroscopic radiography is performed by inserting a catheter into the blood vessels and photographing the course of the blood vessels with contrast medium. Out of such X-ray imaging apparatuses, the apparatuses intended for contrast imaging of cardiovascular vessels that especially move fast strongly need photographing at a high frame rate and high resolution. The sensor device used for the X-ray photographing apparatus is an X-ray image sensor employing an image intensifier (I.I.) or an amorphous silicon (a-Si) thin film transistor (TFT). The I.I. has low spatial resolution due to its structure, which makes it difficult to provide high resolution. The X-ray image sensor employing an a-Si TFT has a problem of being unable to be operated at a high frame rate due to its low current driving capability of the a-Si TFT. Furthermore, in the case of performing X-ray fluoroscopy for a long time period, such as an examination and a treatment with a catheter, keeping the X-ray radiation dose low is strongly required for reducing the amount of radiation that the patient and the practitioner receive. Thus, required specification is an X-ray image sensor that is able to provide a high signal-to-noise ratio (SN ratio) even if the signal quantity is low.
As means for achieving an X-ray image sensor with a high frame rate, an oxide semiconductor TFT with high current driving capability may be conceived. However, the oxide semiconductor TFT has a problem that the oxide semiconductor may metamorphose because of hydrogen included in raw material gas of the hydrogenated amorphous silicon (a-Si:H) thin film that forms the photodiode (hereinafter, referred to as a PD) functioning as a photoelectric conversion element. The deterioration of the oxide semiconductor causes degradation of the TFT characteristics. As a countermeasure, the inventors have proposed an X-ray image sensor disclosed in Japanese Patent Application Laid-Open No. 2015-90957. FIG. 18 is a cross-sectional view illustrating the structure of an image sensor disclosed in Japanese Patent Application Laid-Open No. 2015-90957. The image sensor disclosed in Japanese Patent Application Laid-Open No. 2015-90957 has a structure in which a PD 400 made of hydrogenated a-Si and an oxide semiconductor TFT 300 are formed in this order on a substrate 700 as illustrated in FIG. 18. The image sensor has a structure in which a gas barrier film 730 for preventing hydrogen gas from permeating is placed between the PD 400 and the oxide semiconductor TFT 300. In this structure, the a-Si PD 400 is formed, and then the oxide semiconductor TFT is formed thereon. This allows the oxide semiconductor TFT to be less affected by hydrogen contained in raw material gas of the hydrogenated a-Si. Furthermore, the gas barrier film 730 prevents the hydrogen gas contained in the a-Si PD 400 from diffusing to the oxide semiconductor TFT 300 due to heat treatment at the time of forming the oxide semiconductor TFT 300. This makes it possible to suppress a variation of the characteristics of the oxide semiconductor TFT 300.
Meanwhile, the technique of enhancing the SN ratio of an image sensor includes an active pixel sensor (APS) that is applied to a complementary metal-oxide-semiconductor (CMOS) image sensor. This technique achieves enhancement of the SN ratio by providing each pixel with an amplifier circuit, amplifying a signal from a photoelectric conversion element and outputting the amplified signal.
A method of manufacturing an APS-using image sensor by employing oxide semiconductor TFTs includes a technique disclosed in Japanese Patent Application Laid-Open No. 2011-211171. FIG. 19 is a circuit diagram illustrating a circuit configuration of one pixel of an image sensor disclosed in Japanese Patent Application Laid-Open No. 2011-211171. One pixel of the image sensor is composed of one oxide semiconductor TFT 901 and one PD 902. A gate terminal of the TFT 901 is connected to a selection signal line SEL. A drain terminal of the TFT 901 is connected to an output signal line OUT. A source terminal of the TFT 901 is connected to a photo-sensor reference signal line GND. An anode terminal of the PD 902 is connected to a photodiode reset signal line RST. A cathode terminal of the PD 902 is connected to a back gate of the TFT 901. FIG. 20 is a cross-sectional view illustrating the sectional structure of the image sensor disclosed in Japanese Patent Application Laid-Open No. 2011-211171. The oxide semiconductor TFT disclosed here is of channel etch type and has an inversely staggered structure. In the oxide semiconductor TFT 901, the bottom gate functioning as a main gate is placed at the lower part of the semiconductor layer (substrate side). The oxide semiconductor TFT has a structure in which the oxide semiconductor TFT 901 and the a-Si PD 902 are formed in this order on a substrate 903. A back gate electrode 912 placed in contact with an n-a-Si layer 913 of the a-Si PD is arranged above the channel of the oxide semiconductor TFT 901 via an insulator film 910 and a protection insulating film 911. When the PD 902 is irradiated by light to change the cathode potential, the potential of the back gate electrode 912 also changes. As the potential of the back gate electrode 912 changes, the threshold voltage of the oxide semiconductor TFT 901 also changes. Reading of the signal is performed by switching the selection signal line SEL to a high level to make the oxide semiconductor TFT 901 conductive, and changing the potential of the output signal line OUT that has previously been charged to high voltage. Other examples of the method of modulating the threshold voltage of the transistor by the change of the potential of the PD as disclosed herein are also disclosed in Japanese Patent Application Laid-Open No. 1990-180071 and Japanese Patent Application Laid-Open No. 2009-147056 that employ crystalline silicon as a substrate.